1. Field of the Invention
This invention relates generally to magnetic resonance imaging (MRI) utilizing nuclear magnetic resonance (NMR) phenomena associated with selected NMR nuclei of a patient image volume within an MRI apparatus. It is more particularly directed to method and apparatus for achieving an MRI NMR pulse sequence which combines gradient and spin echo (GRASE) MRI techniques in advantageous ways.
2. Related Prior Art
Over the last ten years or so, commercial MRI systems have become readily available. Some magnetic resonance spectroscopic imaging (MRSI) apparatuses are also now fairly well-known in the art and in use in at least laboratory environments. Similar MRI techniques are utilized in both MRI and MRSI and the term MRI will be collectively used hereinafter to refer to either or both of such techniques and apparatuses.
In conventional MRI apparatus, the relevant patient anatomy is positioned within a predetermined patient imaging volume where a large magnet (e.g, cryogenic, resistive and/or permanent magnet) structure creates a substantially constant and homogeneous magnetic field B.sub.o. Conventional gradient coil structures of various types are also included in the MRI apparatus so as to permit rapid superposition of magnetic gradients with the base magnetic field B.sub.o in the image volume. Typically, these magnetic gradients are labeled G.sub.x, G.sub.y and G.sub.z --indicating gradients oriented along the usual x,y,z Cartesian coordinate system (the B.sub.o field typically being aligned with the z-axis of the same coordinate system). Radio frequency (RF) coils are also tightly RF coupled to the image volume for both transmitting and receiving RF signals to and from the patient tissue nuclei located therewithin.
As is well-known by those in the art, nuclei having an odd number of protons (e.g., hydrogen nuclei) will tend to align their rotating net magnetic moments with the quiescent background magnetic field B.sub.o. However, when subjected to a suitable RF signal at the proper Larmor frequency (proportional to the magnetic field at the site of the nucleus), the rotating net magnetic moments of a substantial proportion of such nuclei may be tilted or nutated away from the quiescent orientation. If subsequently released from such electromagnetic nutation forces, the nuclei will tend to again revert to the quiescent orientation--and will emit characteristic RF signals which can be detected with suitable MRI RF receiving circuits. By subjecting NMR nuclei in a selected image volume to particular sequences of RF nutation pulses and magnetic gradient pulses, NMR RF responses can be detected and processed (e.g., via multi-dimensional Fourier Transformation) so as to yield data representing the spatial distribution of NMR nuclei within the imaged volume. Such data can then be displayed visually where the intensity or color of each pixel or group of pixels in a two-dimensional display represents the NMR nuclei density at a respectively corresponding spatial location within the imaged volume.
Commercially available MRI systems incorporate sophisticated computer control systems for effecting preprogrammed NMR sequences of RF and magnetic gradient pulses for particular types of MRI effects. Virtually any desired NMR sequence can be programmed within the operational limits of the RF and magnetic gradient drivers (e.g., as to magnitudes, rise and fall times, maximum duty cycles, etc). This permits virtually an infinite variety of combinations and permutations of RF and magnetic gradient pulses and many of these possibilities have yet to be explored.
Over the years, many different MRI pulse sequences have been developed and used to successfully image various types of patient tissues. A few of the more well-known MRI pulse sequences are briefly described below: